The tumor microenvironment is increasingly recognized as a major factor influencing not only malignant development, but also resistance to anticancer therapies. It is now evident that the myeloid regulatory cell (MRC) network plays a major role in the failure of antitumor immune responses and therapies to eradicate tumors. Therefore, interventions that can control the effects of tumors on myeloid regulatory cell development are important innovative strategies that should improve the clinical efficacy of novel and conventional therapeutic modalities. Although a crucial role of the MRC in tumor-associated immune suppression has been established, the nature of specific MRC populations and the inter-differentiation pathways of MRC development in cancer remain uncertain. This significantly limits our understanding of the biology of tumor progression and the development of targeted therapeutics to control appearance and differentiation of MRC populations in cancer. Our major hypothesis is that the tumor microenvironment differentially controls emergence of specific myeloid regulatory cell populations which, however, can be redirected by molecular-targeted therapy to support recovery of the antitumor immunity and improve the efficacy of cancer immunotherapy. To test our hypothesis, Aim 1 will characterize myeloid regulatory cell differentiation in the tumor environment in vivo, Aim 2 will determine the mechanisms of myeloid regulatory cell differentiation in the tumor environment, and Aim 3 will define the clinical feasibility of targeting myeloid regulatory cell subsets in the animal tumor models. Successful completion of these studies will 1) provide new insights into the immunobiology of myeloid regulatory cell subsets, including suppressor/regulatory dendritic cells, in cancer by addressing a critical barrier to progress in the field of tumor-mediated immunomodulation, 2) expand an existing paradigm and clinical practice of using FDA-approved pharmacological agents in immunotherapeutic protocols, and 3) support clinical development of molecular targeting of myeloid regulators to maximize the therapeutic value of different therapeutic modalities. Overall impact. The probability that the results of this proposal will contribute considerable levels of new knowledge and exert a potent influence on advancing the field of tumor immunotherapy is quite high. This is due to the many innovative questions and approaches that will be evaluated. This increases the possibility that important information will be collected, and contribute to the pool of information necessary to establish differential targeting of MRC as a highly relevant and useful therapeutic approach that, at least for certain types of cancer, may improve successful outcomes following translation into human clinical trials.